A Rare Case of Thymic Epithelial Tumor: AB-Type Thymoma coexisting with Thymic Adenocarcinoma and Review of literature

Background : Thymic epithelial tumors are rare, with AB-type thymoma coexisting with thymic adenocarcinoma being particularly exceptional. Fewer than 50 cases of combined thymic tumors in the same anterior mediastinal mass have been reported, predominantly involving thymoma combined with squamous cell carcinoma or neuroendocrine tumors. This case report highlights an extremely rare occurrence. Case presentation : A 65-year-old female with a 40-year smoking history presented with a mediastinal mass detected on routine chest computed tomography (CT). Imaging revealed a 6.4cm×4.2cm irregular, lobulated mass in the anterior mediastinum. The patient underwent successful resection via single-port video-assisted thoracoscopic surgery. Pathological examination and immunohistochemical staining confirmed a tumor composed of AB-type thymoma and thymic adenocarcinoma. The thymoma component showed lymphocytes positive for terminal deoxynucleotidyl transferase (TdT), CD5, and CD1a, while the epithelial cells were positive for CK5/6 and P63, with a Ki67 proliferation index of approximately 80% in hostpot areas. The adenocarcinoma component was positive for CK7, CK8/18, CK5/6, CK19, and CEA (occasionally), with a Ki67 proliferation index of approximately 30%. No tumor cells were detected in the anterior superior mediastinal lymph nodes (0/2). The patient has been followed up for 4 months postoperatively and remains disease-free. Conclusions : This case emphasizes the importance of suspecting tumor heterogeneity in mediastinal masses and establishing a definitive diagnosis through pathological and immunohistochemical analysis. As more cases are reported, our understanding of these rare tumors will improve.